In these days, selectively forming an n+-type layer by ion-implanting a dopant such as Si into a semiconductor structure (for example, a AlGaN/GaN structure) that is below an ohmic electrode is proposed in order to reduce an access resistance, on resistance, and the like of a GaN heterojunction field effect transistor.
An example of a structure of this type of the GaN heterojunction field effect transistor (common name of HEMT: High Electron Mobility Transistor) is schematically shown in a cross-sectional view of FIG. 19. As shown in FIG. 19, this HEMT 100 is configured so that an electron transit layer 102 of GaN is formed on a substrate 101 via a buffer layer (not shown), and an electron supply layer 103 of AlGaN forms a heterojunction with an upper surface of the electron transit layer 102. A band gap of the electron transit layer 102 is smaller than that of the electron supply layer 103. A gate electrode 106, a source electrode 107A, and a drain electrode 107B are formed on the electron supply layer 103, and the source electrode 107A and the drain electrode 107B are arranged so that the gate electrode 106 is sandwiched between them. An n-type impurity implantation region 105A is formed in a part of a region including an upper part of the electron transit layer 102 and the electron supply layer, being below the source electrode 107A. Similarity, an n-type impurity implantation region 105B is formed in a part of the region including the upper part of the electron transit layer 102 and the electron supply layer, being below the source electrode 107B. When this HEMT 100 is in on state, a channel of a two-dimensional electron gas layer 110 is formed at a heterojunction interface between the electron supply layer 103 and the electron transit layer 102 and in the neighborhood thereof, whereby a current is passed through this two-dimensional electron gas layer 110. A prior art document on such a GaN HEMT can be, for example, Patent Document 1 (JP 2007-335768 A).